Hello,
I will be building a timber frame pavillion( about 12'w x 16'l). All the main frame components will be 8" x 8" and made from med or hard wood species. The tie beam will be joined to the corner post below the top plate. The joint will be secured with two pegs. What should be a minumum distance that the tie beam( with through tenon) should be below the top plate. I dont want the corner post to eventually split above the joint if the tie beam is too close to the top. Any help would be appreciated.

In the History American Joinery series there are section about the tie connections.
One is "tie at the plate," and one is "tie below the plate."
This series is available from the guild home page. And you can download the sections to view as pdf files on your own computer.
Or you can buy the book for $10 from the guild store.

You may find lots of examples of tie to corner post connections there. One of these may give you your answer.

Are you concerned about uplift? as it is a pavilion.

Jim Rogers

Common rafters? Ridge or no ridge? If a ridge, will it be supported or unsupported?

Welcome Steward;

That is an interesting engineering question thus Tim's questions about the rafters. The roof pitch matters also. It is a small building so relatively small loads. Another factor is the species of wood used for the posts, though I am not sure where to find engineering data for the splitting strength of the different species.

I study historic framing and so I tend to answer people's questions with references to what I have seen done before the early 20th century. Generally this type of framing has a minimum distance of eight inches or more. More than two feet and the posts typically start bending outward. The tie beam joint relies entirely on the pegs unless you can extend the through tenon and add a wedge or two.

If you want to keep the tie beam as high as possible you could use a rare type of tie beam called a head beam and put the tie beam immediately below the plate. Here is what I mean, look at page 252, diagram 6.5:

http://books.google.com/books?id=J9qoVN6...arn&f=false

Jim

Hi Jim, nice book, I should get a copy some time, put it on the book list. It says pages 249-351 are not shown in the link?

Same for me, I couldn't access the page you mention.
Can you show us the diagram?

Jim Rogers

Thanks for the reference Jim. What do you mean by "uplift"?

Timbeal,
Common rafters will be used with a ridge beam. The ridge beam will be supported at each end with a small " queen post" above the tie beam.

Uplift is the effect when a structure that is open on the sides (although not necessarily only such structures) behaves like a wing. The wind blows through and generates lift through the roof. If your structure is only engineered to resist downward force (for example, unpegged stub tenons at the top of the posts into wall plates) then it's possible for the roof to lift clean off!

Steward, it seems you should have no problem with the knee wall cracking above the dropped tie, as there should be no thrust on the walls. The loads will be exerted straight down, into the ridge and the top plate. If you did not have the ridge supported then there would be thrust on the wall.

Yes, uplift is the wind lifting the roof off an open structure.

When the post tenon is cut for a corner post to plate connection normally we don't make it a full width tenon, because if we did it could come right out to the end of the plate. So it is cut back usually 2" from the surface of the post that would be the gable end of the building.

As shown here:



In the above exploded drawing, I have moved the plate up and rotated it so that we could see the tenon and the mortise in the bottom of the plate.

I set the top of the tie to be flush with the bottom of the plate before the housing was cut into the bottom of the plate.

And I followed the same rule that the tenon on the tie beam should be cut back as it is within 2" of the end of the post.

I moved the tie beam back so we could see the tenon. This tenon is now 6" tall instead of a full 8" tenon.

This will allow 2" of relish above the tie beam mortise in the post before the tenon on the post to connect to the plate.

All tenons and mortises are 2" off the face and then 2" thick.

With the post top tenon into the plate 6" it would be hard for the post to split much if the tenon was tight to it's mortise.

But if it did split you could put some fasteners into the post above the tie beam mortise to hold it together. And cover these with some peg ends so that they don't show.



Just my ideas on the subject.

Jim Rogers

Wow! Thanks Jim. Great information and thanks for the diagrams.

Thanks Timbeal. Your information is appreciated.

Jim are you finding this joint is working? The basic structure of the building I'm working on now was not designed in shop and he have had to go to lengths of using splines to keep as much meat in the post as possible in similar cases. In your example we would put a 3-4"x1 1/2 spline in the bottom of the lower beam.

I'll have to go to an existing shed that we put together last September to see if his posts split or not.
When I do I'll take some pictures.

Jim Rogers

Hi;

Sorry my link did not work...it still works for me. I had to go to the source to find an illustration...The Netherlands. I hope you can read Dutch! Not really. You can see the illustration which shows a head beam bent (kopbalkgebinte). While on this site check out the other bent types (gebinten)

http://www.restauratieambacht.be/index.p...&Itemid=451

Jim

Hello,

Kopbalkgebint = head beam bent - (kop, balk, (ge)bint).

Greetings,

Don Wagstaff

I assumed the split was going to be from the top of the through mortise of the top of the post. That's why I suggested fasteners to the side of the post that I did.

Then I was referred to the drawing on page 88 in Jack Sobon's book which shows the split 90° to my assumption.

I now see why there could be pressure there that could split the top of the post.

I has been recommended to me, that the distance from the top of the tie beam to the top of the post should be at least 8". So, I thought I'd pass that on.

To help prevent the post from splitting due to the force of the rafters pushing the plate out, you could put a metal strap on top of the tie beam through the through mortise and up the outside of the post.

Something like this:



However you have said:

Common rafters will be used with a ridge beam. The ridge beam will be supported at each end with a small " queen post" above the tie beam.

Which to me means there will be less thrust force at the plate due to the fact that the common rafters are secured to the ridge beam, than if there was no supported ridge.

Jim Rogers